BRD4 and acute myeloid leukemia: Interestingly, this BRD4 isoform switching significantly reduced survival/proliferation of human MLL-AF9 AML to a similar extent as gRNAs targeting the BRD4 second bromodomain (BD2) or extra-terminal domain (ET), previously shown to be essential for the survival of AMLs driven by MLL rearrangements17 (Fig. 3j, Supplementary Fig. 9a).